As is well known, time division multiplex (TDM) permits a large number of subscribers to be connected to a central communication facility over a single communication path. In telephone systems, the connection between a central office and an individual subscriber has generally been made by a single twisted pair subscriber loop between the central office and the subscriber. Where many subscribers are in the vicinity of a location remote from the central office, it is uneconomical to provide each with a separate subscriber loop. By placing a time division link between a central office and a remote terminal in the vicinity of a group of subscribers, it is possible to eliminate many of the individual subscriber loops. The time division link thereby achieves greater economy, improved capacity and also increases flexibility in the services provided to the subscribers.
Transmission of signals by time division multiplex is accomplished by sampling the information signals from a number of sources at a prescribed rate, converting each sample into a digital code and combining the individual digital codes for transmission over the time division link. Each digital code is assigned a time slot position in a predefined time frame and the information is conveyed in the sequence of frames sent over the link. When received, the digital codes of the individual time slots are separated according to the predefined frame and distributed to the intended destinations. Time division frames may be defined in many ways as long as the selected frame configuration follows the fundamental multiplexing principles. It is essential, however, that both ends of the time division link have the same frame configuration and that the frame processing be synchronized. Otherwise, the information signals cannot be properly organized and distributed.
U.S. Pat. No. 4,125,745 issued to R. E. Steidl, Nov. 14, 1978, discloses a method and apparatus for framing in a time division multiplex communication system in which frame synchronization that avoids reframing on a non-frame bit temporarily containing a preset framing pattern by monitoring for the presence of a repetitive pattern whenever a status signal indicates the absence of frame synchronization. A separate message communication link provided for exchange of system information. The arrangement requires that the framing be predefined and non-changing.
U.S. Pat. No. 4,841,526 issued to J. C. Wilson et al, Jun. 20, 1989, discloses a data communication system in which the size of the time division frame is varied according to the speed, length or error rate to maximize the utilization of the communication link. While the frame size may be adjusted, other parameters defining the frame configuration cannot be changed without disrupting communications.
In many time division multiplex communication systems such as digital loop carrier arrangements, it is often necessary to modify the parameters governing the time division frame configuration. The changes necessitate modifications of all terminals of the time division link. The modifications in a digital loop carrier system made at a central office, however, disrupt communication to the other terminals of a system. Unless the framing configuration of the remote terminal is changed, the terminal is not able to process any information. Thus, it is not possible to communicate the framing changes to the remote terminals over the time division multiplex link unless the system is arranged so that frame change information can be sent priorly or there is an independent communication path adapted to convey such information. Prior transmission of frame change information, however, must be accurately synchronized with the change in frame configuration. Otherwise, there will be a time period during which all signal exchange between the central office and the remote terminal will be disrupted.
In the absence of an independent communication path for transfer of terminal configuration information, physical access to the terminals of the system is required. Often, physical access is not possible as in satellite terminals or is too costly as in digital loop multiplex systems with many remotely located terminals. It is an object of the invention to provide improved frame changeover in a time division communication system so that framing parameters may be altered without the need for physical access to all terminals or the use of a communication link in addition to the information conveying time division link.